Internal-combustion engines – Combustion chamber means having fuel injection only – Combustible mixture stratification means
Reexamination Certificate
2001-08-23
2003-12-30
Solis, Erick (Department: 3747)
Internal-combustion engines
Combustion chamber means having fuel injection only
Combustible mixture stratification means
C123S299000
Reexamination Certificate
active
06668789
ABSTRACT:
FIELD OF THE INVENTION
This disclosure concerns an invention relating generally to reduction of emissions such as particulates and NO
x
in internal combustion engines, and more specifically to emissions reduction in compression ignition (CI or diesel) engines.
BACKGROUND OF THE INVENTION
Common pollutants arising from the use of internal combustion engines are nitrogen oxides (commonly denoted NO
x
) and particulates (also known simply as “soot”). NO
x
is generally associated with high-temperature engine conditions, and may be reduced by use of measures such as exhaust gas recirculation (EGR), wherein the engine intake air is diluted with relatively inert exhaust gas (generally after cooling the exhaust gas). This reduces the oxygen in the combustion region and obtains a reduction in maximum combustion temperature, thereby deterring NO
x
formation. Particulates include a variety of matter such as elemental carbon, heavy hydrocarbons, hydrated sulfuric acid, and other large molecules, and are generally associated with incomplete combustion. Particulates can be reduced by increasing combustion and/or exhaust temperatures, or by providing more oxygen to promote oxidation of the soot particles. Unfortunately, measures which reduce NO
x
tend to increase particulate emissions, and measures which reduce particulates tend to increase NO
x
emissions, resulting in what is often termed the “soot-NO
x
tradeoff”.
At the time of this writing, the diesel engine industry is facing stringent emissions legislation in the United States, and is struggling to find methods to meet government-imposed NO
x
and soot targets for the years 2002-2004 and even more strict standards to be phased in starting in 2007. One measure under consideration is use of exhaust after-treatment (e.g., particulate traps) for soot emissions control in both heavy-duty truck and automotive diesel engines. However, in order to meet mandated durability standards (e.g., 50,000 to 100,000 miles), the soot trap must be periodically regenerated (the trapped soot must be periodically re-burned). This requires considerable expense and complexity, since typically additional fuel must be mixed and ignited in the exhaust stream in order to oxidize the accumulated particulate deposits.
Apart from studies directed to after-treatment, there has also been intense interest in the more fundamental issue of how to reduce NO
x
and particulates generation from the combustion process and thereby obtain cleaner “engine out” emissions (i.e., emissions directly exiting the engine, prior to exhaust after-treatment or similar measures). Studies in this area relate to shaping combustion chambers, timing the fuel injection, tailoring the injection rate during injection so as to meet desired emissions standards, or modifying the mode of injection (e.g, modifying the injection spray pattern). One field of study relates to premixing methodologies, wherein the object is to attain more complete mixing of fuel and air in order to simultaneously reduce soot and NO
x
emissions. In diesel engines, the object of premixing methodologies is to move away from the diffusion burning mechanism which drives diesel combustion, and instead attempt to attain premixed burning. In diffusion burning, the oxidant (fuel) is provided to the oxidizer (air) with mixing and combustion occurring simultaneously. The fuel droplets within an injected spray plume have an outer reaction zone surrounding a fuel core which diminishes in size as it is consumed, and high soot production occurs at the high-temperature, fuel-rich spray core. In contrast, premixed burning mixes fuel and air prior to burning, and the more thorough mixing results in less soot production. Premixing may be performed by a number of different measures, such as by use of fumigation (injection of fuel into the intake airstream prior to its entry into the engine), and/or direct injection of a fuel charge relatively far before top dead center so that piston motion and convection within the cylinder result in greater mixing.
One promising diesel premixing technology is HCCI (Homogeneous Charge Compression Ignition), which has the objective of causing initial ignition of a lean, highly premixed air-fuel mixture near top dead center (near the end of the compression stroke or the beginning of the power stroke). An extensive discussion on HCCI and similar premixing techniques is provided in U.S. Pat. No. 6,230,683 to zur Loye et al., and U.S. Pat. No. 5,832,880 to Dickey and U.S. Pat. No. 6,213,086 to Chmela et al. also contain useful background information. The charge is thus said to be “homogeneous” in HCCI because it is (at least theoretically) highly and evenly mixed with the air in the cylinder. Ignition is then initiated by autoignition, i.e., thermodynamic ignition via compression heating. The objective is to use autoignition of the lean mix to provide significantly lower combustion chamber temperatures, thereby diminishing NO
x
production (which thrives at high temperature). In contrast, a richer mixture (such as that necessary for flame propagation from the spark in an SI engine) will burn more quickly at greater temperature, and therefore may result in greater NO
x
production.
As the foregoing references note, while the HCCI process might be beneficially implemented, it is also hard to accomplish owing to the difficulties in igniting the lean mix and/or controlling the start of ignition. Combustion in an SI engine is readily initiated by the spark, with premixed burning occurring afterward; similarly, combustion in a conventional CI engine is initiated by fuel injection near top dead center (following compression) when thermodynamic conditions for autoignition are favorable, with diffusion burning occurring afterward. However, HCCI does not utilize a spark, nor is it desirable to use the rich mixture needed for effective use of a spark. It is also difficult for HCCI to achieve a homogeneous charge or premixed burning if injection near top dead center (during or after compression) is used, since there is less time for mixing to occur. Thus, a key area of study in the HCCI field is how and when to efficiently initiate ignition, and ignition and timing problems are the primary reason why HCCI has not attained widespread use.
Multiple injection, also called split injection, pilot injection, and post injection, has also been a proposed method for NO
x
and particulate emissions reduction in diesel engines (see, e.g., Tow, T., Pierpont, A. and Reitz, R. D. “Reducing Particulates and NO
x
Emissions by Using Multiple Injections in a Heavy Duty 0.1. Diesel Engine, ” SAE Paper 940897, SAE Transactions, Vol. 103, Section 3, Journal of Engines, pp. 1403-1417, 1994). A multiple injection engine varies from the standard “single injection” engine in that the direct injection of a single fuel charge during the combustion cycle is replaced by direct injection of several fuel charges spaced over time, with less fuel being used per injection so that the total amount of fuel finally injected per cycle is comparable to that used in single injection. The multiple injections take place around top dead center (after compression), and burning occurs in a diffusion mode wherein each charge burns upon injection, without premixing. Thus, the division of the “standard” single injected charge into several smaller discrete charges spaced over time results in steady and more complete burning of the injected fuel plumes with more evenly maintained combustion temperature, which helps decrease emissions. While multiple injection is not in common use at the time of this writing, engines using the multiple injection concept are now in production or under development in Europe, Japan and the United States.
Further, while multiple injection will assist the diesel engine industry in meeting emissions goals, it unfortunately does not appear to be a complete solution: it does not by itself decrease emissions to the minimum levels desired. There is thus a significant need for methods and apparata which assist in compression ignition or diesel
Marriott Craig D.
Reitz Rolf D.
DeWitt Ross & Stevens S.C.
Fieschko, Esq. Craig A.
Solis Erick
Wisconsin Alumni Research Foundation
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